Wave/Current–Structure–Seabed Interactions Around Offshore Foundations

The rapid expansion of the offshore energy infrastructure, including wind turbines, subsea pipelines, and marine platforms, has underscored the critical need to overcome the challenges presented by harsh marine environments. These structures are exposed to complex interactions among waves, currents, seabed dynamics, and structural vibrations, which can trigger geotechnical instabilities, sediment scouring, and fatigue failure. This Special Issue, titled “Wave/Current–Structure–Seabed Interactions Around Offshore Foundations”, brings together cutting-edge research to deepen our understanding of these multifaceted interactions and their implications for the design and safety of offshore engineering. The 11 articles featured here employ experimental, numerical, and theoretical approaches to investigate pivotal topics such as scour dynamics, seabed liquefaction, hydrodynamic loading, and multi-physics coupling effects. Below, we synthesize the novel contributions of each study and discuss their significance for future research and industrial applications.

Ref. [1] conducted an experimental study on local scour patterns around complex multi-pile foundations under steady currents. Employing a meticulously designed 5 × 5 pile group model, they systematically investigated the relationship between flow dynamics and scour depth evolution. Their results demonstrate that the maximum scour depth exhibits strong dependence on flow intensity, and the distribution of scour depths among piles differs at different flow intensities. This research elucidates scour mechanisms in pile groups by demonstrating that hydrodynamic interactions induce significant shielding effects—progressively reducing scour depths downstream—and trigger a critical transition from isolated-pile-like scour patterns to group-dominated behavior at elevated flow intensities. The findings indicate that pile group scour is governed by emergent spatial dependencies rather than superposition of single-pile responses, thereby challenging conventional extrapolation approaches in foundation design. These insights provide a mechanistic basis for advancing predictive models and optimizing risk mitigation strategies in marine infrastructure. Ref. [2] pioneered wave-current-vibration flume experiments to quantify monopile vibration effects on local scour dynamics. The equilibrium scour depths in clear-water conditions are reduced by pile vibration while triggering non-monotonic depth variations under live-bed scour conditions near the critical Shields parameter. Crucially, higher vibration frequencies predominantly suppress the scour depth, whereas increased amplitudes expand the width of the scour hole. This behavior stems from vibration-induced sediment ratcheting convection and densification, which enhance scour hole backfilling. Their predictive model—the first to integrate vibration intensity with hydrodynamic parameters (Froude number, Keulegan–Carpenter number, and velocity ratio)—provides a critical design framework for offshore wind turbine foundations under dynamic loading. Ref. [3] addressed pipeline instability challenges on “sand wave” seabeds, where natural undulations create hazardous free spans that are prone to Vortex-Induced Vibration (VIV) fatigue. Through coupled geotechnical–hydrodynamic modeling, they evaluated post-lay trenching as a mitigation strategy. A comparative fatigue assessment of trenched vs. untrenched pipelines across realistic sand wave morphologies was performed. It was shown that targeted trenching for post-lay rectification (trenching to a depth of 1 m) significantly reduces stress ranges and extends fatigue life, meeting both Ultimate Limit State (ULS) and Fatigue Limit State (FLS) requirements. The study provides a methodology for pipeline route planning and intervention design on morphologically active seabeds.

Ref. [4] presented a significant advancement in modeling wave-induced seabed liquefaction around buried pipelines by adopting a non-Darcy flow-based 2D finite element model. Their simulations reveal that nonlinear flow resistance fundamentally alters the pore pressure distribution, resulting in a shallower yet broader liquefied zone compared with conventional predictions. Crucially, the pipeline itself exerts a “liquefaction shielding effect,” suppressing seabed liquefaction and constraining the extent of the liquefied zone relative to far-field conditions. These findings establish a more realistic framework for assessing pipeline flotation risks and evaluating mitigation strategies such as increased burial depth or soil densification in liquefaction-prone environments. Ref. [5] conducted pioneering numerical investigations into tidal bore-induced seabed liquefaction—an extreme but often neglected hazard. By integrating generalized Biot theory with advanced integral transform methods, they simulated the rapid propagation of a high-energy tidal bore through a shallow estuary and its destabilizing effects on sandy riverbeds. Their results demonstrate that the abrupt pressure gradient preceding the bore front induces instantaneous liquefaction in the upper seabed layer within seconds, with the liquefaction depth and intensity being governed by the sediment permeability, saturation level, and bore height. This work delivers critical predictive relationships for assessing liquefaction risks under transient hydraulic loading conditions. Ref. [6] introduced an innovative hybrid Discrete Element Method (DEM)–Finite Element Method (FEM) framework to elucidate the mechanics of lateral pipe–soil interaction on sloping seabeds. The DEM component accurately captures granular sediment behavior, including soil upheaval, particle flow, and localized densification, while the FEM efficiently models the structural pipe response. Their simulations uncover how the seabed slope dictates the failure wedge geometry and lateral resistance during pipe movement, while also revealing nuanced mechanisms such as post-breakout soil resistance evolution and the influence of pipe rotation on deeper soil mobilization. This methodology provides unprecedented insights for refining the large-scale pipeline–soil interaction models that are used in slope stability design.

Ref. [7] developed an advanced method for predicting the fatigue life of free-spanning pipelines that are subject to Vortex-Induced Vibration (VIV), integrating three key components: (1) dimensionless vibration amplitude A/D-V_r relationships for a wall-free circular cylinder with low mass damping, (2) beam-theory stress distribution analysis, and (3) high-cycle (N > 10⁷) S-N curves for high-strength steel pipelines with cathodic protection under seawater environments. Their parametric studies revealed nonlinear fatigue life-flow velocity relationships that challenge conventional linear assumptions in pipeline design. Ref. [8] employed the novel technique of transparent soil combined with Particle Image Velocimetry (PIV) to visualize and quantify the complex coupled thermal–hydraulic processes around a buried pipeline. This non-invasive method allows for unprecedented observation of the convective flow field and temperature distribution in the soil annulus surrounding an actively heated or cooled pipeline. Key findings demonstrate that natural convection currents become the dominant heat transfer mechanism at shallow burial depths, significantly altering isotherm shapes and reducing thermal gradients compared with pure conductive models. The study quantifies the convection strength relative to the burial depth, heat flux, and soil properties, offering valuable data to improve the accuracy of numerical models predicting pipeline temperature dissipation and long-term impacts on seabed thermal regimes. Ref. [9] employed Discrete DEM simulations with realistically shaped, crushable particle models to elucidate the micromechanical mechanisms behind strength/stiffness anisotropy in calcareous sands. Their triaxial shear simulations with varying particle breakage ratios revealed that contact force redistribution from vertical to horizontal directions during particle crushing weakens deviatoric stress networks, fundamentally altering the peak strength, deformation modulus, and failure patterns. These micromechanical insights provide essential foundations for developing anisotropic constitutive models to enhance offshore foundation safety in calcareous environments.

Ref. [10] performed high-fidelity CFD simulations to unravel the complex hydrodynamics of semi-submersible platforms for floating offshore wind turbines (FOWTs). Their study reveals that submerged braces and pontoons contribute substantially to the total drag forces at high Reynolds numbers—a finding that challenges conventional models focusing solely on column effects. The research provides quantitative relationships between drag coefficients, Reynolds numbers, and platform orientation, offering critical data for optimizing structural design and mooring systems to enhance storm survivability. Ref. [11] explored the innovative synergy of offshore wind energy infrastructure with aquaculture by numerically analyzing the hydrodynamic interactions between wind turbine monopile foundations and pontoon raft aquaculture facilities (PRAFs). Using CFD modeling validated against physical experiments, they assess how the raft arrays modify wave propagation, reflection, and dissipation around the monopile and, reciprocally, how the monopile influences raft motions and mooring tensions. A major finding is that multi-row raft configurations positioned upstream of the monopile effectively act as wave dampers, reducing wave heights at the monopile by up to 20% depending on the row number, wave period, and raft submergence. Concurrently, the structural presence of the monopile provides shielding, reducing the peak mooring tension in multi-row PRAF arrangements by up to 73% compared with single-row PRAF arrangements. This research provides hydrodynamic evidence supporting the technical feasibility and mutual benefits (reduced structural loads, sheltered aquaculture conditions) of integrated offshore multi-use platforms.

This collection of studies elucidates the intricate multi-physics phenomena governing wave/current–structure–seabed interactions in offshore foundation systems, while reporting pioneering novel methodologies to enhance marine infrastructure resilience. Through systematic integration of experimental investigations, numerical modeling breakthroughs, and theoretical advancements—spanning scour dynamics, seabed liquefaction mechanisms, coupled hydrodynamic loading, and sustainable design paradigms—this Special Issue makes substantive contributions to the fundamental science of offshore engineering. The rigorous findings presented not only elevate predictive frameworks for geohazard evaluation but also propel the development of innovative engineering solutions. These advances are particularly timely and vital as the offshore industry confronts the dual challenges of expanding marine resource exploitation and increasingly stringent safety requirements in complex marine environments.